Lithium ion secondary batteries have higher energy densities than nickel-cadmium secondary batteries and nickel hydrogen secondary batteries and thus are widely used as power units of notebook computers and mobile phones. While electric cars and hybrid cars have been developed as environment-friendly cars, the application of lithium ion secondary batteries as a kind of nonaqueous secondary batteries to power units for automobiles have been discussed.
In general, a lithium ion secondary battery is constructed by housing a positive electrode in which a current collector is coated with positive electrode active material particles using a binder, a negative electrode in which a current collector is similarly coated with negative electrode active material particles using the binder, and an electrolyte layer arranged in the center thereof in a battery case. As the negative electrode active material of such a lithium ion secondary battery, a carbon material such as graphite as shown in PTL 1 or a material forming an alloy (hereinafter, an alloy based material) of silicon, tin and the like with lithium as shown in PTLs 2, 3 is used. The alloy based material is characterized in that the lithium occlusion amount, that is, the charged capacity thereof is larger than that of the carbon material and enables a larger capacity of a battery. However, the alloy based material undergoes large volume changes accompanying the charge and discharge and thus, particles bonded by a binder are more likely to be disconnected. Particles whose connection to other particles is disconnected lose an electron conduction path and no longer contribute to subsequent charges and discharges. That is, a problem of the alloy based material is that the reduction in capacity accompanying the charge and discharge cycle is large.
To inhibit the reduction in capacity of a negative electrode using the alloy based material, it is effective to prevent an electron conduction path between active material particles from being lost due to volume changes. As such a technology, for example, PTL 4 discloses a lithium secondary battery including a negative electrode in which a negative electrode active material layer containing negative electrode active material particles and a negative electrode binder is formed on the surface of a negative electrode current collector, a positive electrode, and a nonaqueous electrolyte, wherein mixed particles obtained by mixing a particle A having a particle distribution of the presence of 60% by volume or more in the range of 3 μm or more and 6 μm or less in median diameter (D50) and 2 μm or more and 7 μm or less in particle size and containing silicon and/or a silicon alloy and a particle B having a particle distribution of the presence of 60% by volume or more in the range of 9 μm or more and 15 μm or less in median diameter (D50) and 7 μm or more and 17 μm or less in particle size and containing silicon and/or a silicon alloy in the range of the weight ratio (particle A:particle B) 10:90 to 25:75 are used as the negative electrode active material particles and a polyimide resin having a specific structure as the negative electrode binder. According to PTL 4, degradation of current collecting properties inside the negative electrode accompanying the passage of charge and discharge cycles is suppressed by a high level of adhesiveness of the polyimide resin and silicon so that a lithium secondary battery having a high energy density and superior in charge and discharge cycle characteristics can apparently be produced.
PTL 5 discloses a negative electrode material created by a conductivity improver being contained in a composite material created by combining particles containing a solid phase A made mainly of Si and a solid phase B made of an intermetallic compound of at least one metal selected from Cr and Ti and Si with a carbon material. According to PTL 5, the particle shape is retained when the volume expands and when used in a lithium ion secondary battery, a negative electrode material superior in repeated charge and discharge cycle characteristics while retaining a high capacity and superior in charge and discharge efficiency in the initial cycle is apparently provided.
PTL 6 discloses a negative electrode material for a lithium ion secondary battery containing negative electrode active material powder containing silicon powder in which the surface of particles is coated with a carbon film, conductive carbon powder existing among particles of the negative electrode active material powder and having a smaller average particle size of primary particles than that of the negative electrode active material powder, and a conductive carbon fiber coexisting with the conductive carbon powder among particles of the negative electrode active material powder and having a thinner average diameter of fiber than the average particle size of primary particles of the negative electrode active material powder and the conductive carbon powder and a longer average length of fiber than the average particle size of primary particles of the conductive carbon powder. According to PTL 6, conduction between the negative electrode active material and the current collector is maintained even if volume changes of silicon particles arise accompanying the charge and discharge and therefore, degradation of battery characteristics caused by volume changes of silicon particles accompanying the charge and discharge is apparently suppressed.
PTL 7 discloses a lithium secondary battery negative electrode mixture containing a negative electrode active material (A) and a binder (B), wherein the negative electrode active material (A) contains composite particles made of silicon containing particles containing an alloy, oxide, nitride, or carbide of silicon capable of occluding/releasing lithium ions and a resin carbon material surrounding the silicon containing particles and a silicon containing network structure bound to the surface of the composite particles and made of nano-fibers and/or nano-tubes surrounding the composite particles and the binder (B) contains polyimide. According to PTL 7, pulverization of the negative electrode active material caused by the charge and discharge cycle is suppressed by an extremely high synergistic effect of the negative electrode active material and the binder and also adhesiveness between nano-fibers and/or nano-tubes and composite particles is maintained and therefore, the negative electrode active material is inhibited from dropping from the electrode and a lithium secondary battery negative electrode mixture exhibiting hitherto unseen superior charge and discharge cycle characteristics, a lithium secondary battery negative electrode, and a lithium secondary battery using the negative electrode are apparently provided.
PTL 8 discloses a negative electrode material for a nonaqueous electrolyte secondary battery made of a coated mixture in which each of (A) particles having a structure in which silicon oxide particles represented by the general formula SiOx (1.0≤x<1.1) or microcrystals of silicon are dispersed in a silicon based compound and (B) composite particles in which the surface of an Si particle is coated with a carbon nano-tube, carbon nano-fiber, or carbon fiber is further coated with a carbon film. According to PTL 8, a lithium ion secondary battery and an electrochemical capacitor achieving high initial charge and discharge efficiency and high capacities and superior in cycle characteristics can apparently be obtained.